[Haskell-cafe] Can not use ST monad with polymorphic function

Dmitry Kulagin dmitry.kulagin at gmail.com
Wed Nov 28 18:52:30 CET 2012

Hi Cafe,

I try to implement some sort of monadic fold, where traversing is
polymorphic over monad type.
The problem is that the code below does not compile. It works with any
monad except for ST.
I suspect that monomorphism is at work here, but it is unclear for me how
to change the code to make it work with ST.

fold :: Monad m => (Int -> m ()) -> m ()
fold f = mapM_ f [0..20]

selectFold :: Monad m => String -> IO ((Int -> m ()) -> m ())
selectFold method = do
    -- in real program I'd like to choose between
    -- different fold methods, based on some IO context
    return fold

useFold :: Monad m => ((Int -> m ()) -> m ()) -> m ()
useFold fold' = fold' f
    where f _ = return () -- some trivial iterator

main = do
    fold'' <- selectFold "some-method-id"
    print $ runST $ useFold fold''

Thank you!
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